Title: Epigenetic Regulation of Germ Cell Differentiation from a Stem Cell Lineage Project Summary: Many human diseases are caused by imbalances between self-renewal/proliferation versus differentiation of adult stem cells. Therefore, it is critical to characterize the molecular and cellular mechanisms that regulate stem cell function to understand the diseases such as cancers and tissue degeneration. Additionally, to utilize stem cells in therapeutic applications, it is critical to understand the mechanisms that maintain stem cells, regulate proper proliferation of progenitor cells and program cellular differentiation in stem cell lineages. Stem cell-based regenerative medicine also holds promise for diseases such as cancers, neurodegeneration, muscle dystrophy, diabetes, and infertility. A broad definition of epigenetic mechanisms refers to DNA-associated factors that change gene expression without altering the primary DNA sequences. Epigenetic mechanisms play crucial roles in defining stem cell identity and regulating stem cell activity. Among known epigenetic regulators, the Polycomb group (PcG) transcriptional repressive proteins have critical functions in multiple stem cell lineages across different species from Drosophila to mammals. In addition to their well-characterized functions in silencing gene expression, studies from many labs have shown other roles of PcG in areas such as DNA replication and cell cycle progression. My lab uses the Drosophila male germline stem cell (GSC) lineage as a model system to investigate how epigenetic mechanisms regulate stem cell maintenance, proliferation, differentiation, and dedifferentiation. Our previous studies have shed light on epigenetic mechanisms in regulating multiple events in GSC lineage including GSC maintenance and germ cell differentiation. Our recent work reveals under-appreciated functions of DNA replication and potentially new roles of epigenetic regulators such as Polycomb (Pc). We propose to use well-established and newly developed methods in molecular genetics, cell biology, biophysics and genomics to provide an in-depth and systematic understanding of how key epigenetic players regulate stem cells in vivo. Successful completion of the proposed studies will have a broad impact on stem cell biology, germ cell biology, DNA replication, chromatin biology, cancer biology, reproductive biology, and regenerative medicine.